Metals and Materials International

This study examines the friction and wear of ceramic matrix composites designed for use in automotive brake discs. The composites are produced by reinforcing a SiC matrix with carbon fibers using a liquid silicon infiltration method. C/C-SiC composites with two different compositions are fabricated to examine the compositional effect on the tribological properties. The tribological properties are evaluated using a scale dynamometer with a low-steel type friction material. The results show that the coefficient of friction is determined by the composition of the composite, which affects the propensity of friction film formation on the disc surface. A stable friction film on the disc surface also improves the wear resistance by diminishing the abrasive action of the disc. On the other hand, the friction film formation on the disc is affected by the applied pressure, and stable films are obtained at high pressures. This trend is prominent with discs with high Si content. However, both C/C/-SiC composites show superior performance in terms of the friction force oscillation, which is closely related to brake-induced vibration.

The present work studies the synthesis and consolidation of γ-Ni-Fe nanoalloy powder by the mechano-chemical process comprising high-energy ball-milling of NiO-Fe2O3 powder and a subsequent hydrogen reduction process. To examine the formation mechanism of the nanoalloy powder, the effect of the oxide powder char-acteristics on the reduction process and alloying was investigated by varying the ball-milling time. The reduction process and the alloying of the γ-Ni-Fe phase proved to accelerate as the ball-milling time increased. However, prolonged milling (for 30 hours) retarded the reduction of Fe2O3 as well as the alloying process. The densification process of the Ni-Fe nanoalloy powder strongly depended on the degree of agglomeration which results in enhancing homogeneous sintering. The limited densification of the nanoalloy powder originates from the high degree of particle agglomeration. While intra-agglomerate porosity is easily eliminated in the course of sintering, it is found to resist densification. The limitation of the sintered density could be overcome by increasing the green density of the powder compacts. Full density was achieved by starting with a green density of 72% theoretical density.

In this study, the high-temperature corrosion resistance of plasma-sprayed ceramic oxide coatings has been evaluated in a LiCl-Li2O molten salt under an oxidizing environment. Al2O3 and YSZ coatings were manufactured by atmospheric plasma spraying onto a Ni alloy substrate. Both the plasma-sprayed Al2O3 and YSZ coatings had a typical splat quenched microstructure which contained various types of defects, including incompletely filled pores, inter-splat pores and intra-splat microcracks. Corrosion resistance was evaluated by the thickness reduction of the coating as a function of the immersion time in the LiCl-Li2O molten salt at a temperature of 650 °C. A linear corrosion kinetic was found for the Al2O3 coating, while no thickness variation with time occurred for the YSZ coating. The ceramic oxide coatings were reacted with LiCl-Li2O molten salt to form a porous reaction layer of LiAl, Li5AlO4 and LiAl5O8 for the Al2O3 coating and a dense reaction layer of non-crystalline phase for the YSZ coating. The reaction products were also formed along the inside coating of the porous channel. The superior corrosion resistance of the YSZ coating was attributed to the formation of a dense protective oxide layer of non-crystalline reaction products on the surface and at the inter-splat pores of the coating.

Nanocrystalline TiO2 layers were synthesized onto a mica substrate using a room temperature homogeneous nucleation method. The synthesis was controlled by optimizing the precipitants, the pH, the reaction temperature, the reaction time, and the stirring rate. The TiO2 layers were characterized by photospectrometry, X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM). The layers were well synthesized even at room temperature, which resulted in pigments having a strong interference color.

The influence of process parameters on geometry and the quality of amorphous ribbons produced by the Chill Block Melt Spinning (CBMS) method from, Fe82B18, Fe79.7V2.3B18, Fe77.5V4.5B18 and (Co,Ni,Fe)73(B,Si)27 (in at%) alloys was studied experimentally and theoretically. The assessment of the forces, acting in the melt-substrate contact zone, showed that at CBMS the puddle is shaped by a hydrodynamic mechanism. Thus, by implementing the ideas of the dimensional analysis and solving the equations of heat and momentum transfer, it was possible to obtain a general functional dependency between the process parameters and the geometry of the final product at CBMS (amorphous, or microcrystalline, ribbon). A method for the evaluation from the experimental data of the inherent to the dimensional analysis unknown parameters was developed and tested. The functional dependency was used: (1) to predict the ribbon geometry as a function of the process parameters; (2) to evaluate the cooling rates achieved during the processing of the amorphous ribbons.

The binary trialuminides typically crystallize with the tetragonal D022 (or DO23) structures and frequently exist as line compounds, making it very difficult to produce them as single-phase material. As a result of their low symmetry, ordered tetragonal structures, these compounds show such limited ductility at and immediately above room temperature as to find no useful engineering application. The compound Al3Ti is known to deform by ordered twinning at ambient temperature, which does not disturb the D022 symmetry of the lattice during deformation, but leads to only four potential deformation systems, which is insufficient for the generalized von Mises plasticity criterion. Recent research effort has moved to improving the ductility of the trialuminides by transforming their tetragonal (D022/D023) crystal structures into the closely-related ordered cubic Ll2 structure, in the hope that the increased number of independent slip systems in the cubic structure will enable the alloys to deform more easily. Significant ductility in compression, and measurable plastic strain on the tensile side of bend bars, have been reported, especially in Cr and Mn-modified Ll2 alloys. However, notwithstanding these hopeful signs in the Ll2 trialuminides, these cubic alloys remain uniformly brittle in tension at room temperature. At present, the brittle behaviour of the Ll2 trialuminides appears to be intrinsic to their nature, with little scope for improvement by microstructural modification. The controversy assocated with room temperature dislocation dissociation in the Ll2 trialuminides has been concluded that the superdislocations on 111 planes are APB-dissociated pairs rather than SISF-coupled partials. In attempting to identify new approaches to overcoming the brittleness of trialuminide-based alloys, it is worth noting potential advantages of multiphase alloys over single phase alloys. The development of fine duplex microstructures, by combining judicious alloying with controlled thermal or thermo-mechanical treatments, appears to offer promise for enhancing the ductility of brittle monolithic alloys. Given this evidence, it is suggested that the design and development of multiphase or duplex microstructures for trialuminide-based alloys may provide an approach of interest in providing the ductility and/or toughness of such alloys.

In this paper, the influence of weld corrosion on the mechanical behavior of a welded joint pipeline was investigated using corrosion and mechanical simulations simultaneously. In the corrosion simulation, the modeling results (i.e., the corrosion potential and current density) revealed that the welded joint is preferentially corroded and the corrosion rate is higher in the outside environment due to the severe corrosion factors. The increase in corrosion degradation according to the operation time increases the stress concentration on the welded joint, indicating that the failure risk of the welded joint is increased with increasing corrosion degradation. These results can be used to evaluate the lifetime of welded joints exposed to corrosion and suggest guidelines for the maintenance of structures.

This work deals with the problems of structural regeneration by thermal restoration treatment (TRT). These include the lack of a structural sign showing that TRT is possible, a consensus on TRT modes, the data on the necessary relaxation depth of residual stresses, or criteria of structural restoration. Performing a TRT without solving these problems may deteriorate the properties of steel or even accelerate its destruction. With this in view, the purpose of this work is to determine experimentally how the residual stress state changes under thermal and mechanical loads in order to specify the signs of the restoration of structure and structural stability. The object of this research is unused 12Cr1MoV steel that has been aged naturally for 13 years. Using X-ray dosimetry with X-ray spectral analysis, we study the distribution of internal residual stresses of the first kind during the repeated hot deformation. After repeated thermal deformation, the sample under study transforms from a viscoelastic Maxwell material into a Kelvin-Voigt material, which facilitates structural stabilization. A sign of this is the relaxation limit increase, prevention of continuous decay of an α-solid solution of iron and restoration of the lattice parameter.